1. Field of the Invention
The present invention relates to a composite lathe including: a first and a second spindle headstock disposed on a bed; a tool post disposed between the first and second spindle headstocks to be movable in an X-axis direction; and a third spindle disposed on the bed to be movable in the X-axis direction, a Y-axis direction, and a Z-axis direction.
2. Description of the Related Art
An example of a composite lathe of this type is structured such that a spindle headstock is disposed on a bed, and a lower tool post and an upper tool post are disposed on an outer lower side and an inner upper side of a vertical line going through an axis of the spindle headstock respectively to be movable in an X-axis direction and a Z-axis direction.
This composite lathe adopts a structure such that an X-axis guide by which the upper tool post is supported to be movable in the X-axis direction and an X-axis guide by which the lower tool post is supported to be movable in the X-axis direction are disposed on the same plane (see, for example, Patent document 1).
[Patent document 1] Japanese Patent Application Laid-open No. He 6-106438
Incidentally, in the structure in which the upper tool post and the lower tool post are moved on the same plane as in the aforesaid conventional lathe, movement areas of cutting points of the upper and lower tool posts are limited, inhibiting each of them from moving into the movement area of the other, resulting in limited machinability.
Here, for example, in order to extend the movement area of the cutting point of the upper tool post to an area across the cutting point of the lower tool post, it is necessary to make the upper tool post overhang from a surface of the X-axis guide movably supporting the upper post. However, such a structure tends to lower supporting stiffness of the upper tool post, giving an adverse influence to machining accuracy.